Use go build tags to enable/disable metrics publication

[raulb]

Is your feature request related to a problem? Please describe.

With the use of go build tags, we could build with metrics only when distributing the CLI via our release process.

Describe the solution you'd like

1. Have a build tag such as // +build metrics in those parts where we emit metrics.
2. Update goreleaser.yml to use this flag.
3. Consider removing the option to silence metrics via PUBLISH_METRICS=false

Describe alternatives you've considered

Leave implementation as is.

Additional context

This article explains very well how to make use of these build tags: https://www.digitalocean.com/community/tutorials/customizing-go-binaries-with-build-tags